G-protein coupled receptors (GPCRs) regulate critical physiological processes such as neurotransmission, growth, and differentiation, and are also targeted by a large percentage of therapeutic agents and recreational drugs. Heterotrimeric guanine nucleotide binding proteins (G-proteins) act as molecular switches in GPCR pathways, in which the activity of the Ga subunit is determined by whether GTP or GDP is bound. Relative levels of GTP and GDP occupancy reflect the balance between nucleotide exchange and GTP hydrolysis. The former reaction is catalyzed by ligand activation of GPCRs coupled to heterotrimers, while the latter reaction is catalyzed by GTPase activating proteins, or GAPS. Regulators of G-protein Signaling (RGS) are diverse proteins that act as GAPs for Ga subunits. A subfamily of RGS proteins (RGS6, 7, 9, 11) contains a Gg-like (GGL) domain which mediates binding to b subunits. Therefore, in addition to interaction with GTP-Ga subunits through the RGS domain, GGL domain containing RGS (GGL-RGS) proteins may also associate with GDP-Ga subunits to promote receptor coupling. This project is designed to test the hypothesis that GGL-RGS proteins regulate both receptor coupling of GDP-Ga and GTP hydrolysis of GTP-Ga through the GGL and RGS domains, respectively, and that the alpha selectivity of the two domains determine the overall effect of GGL-RGS proteins on GPCR signaling. To test this hypothesis, the Ga selectivity of both the GUL and RGS domains will be determined, the molecular determinants of this selectivity will be defined, and wild type and mutant GGL-RGS constructs will be compared for their ability to regulate GPCR activation of effector pathways.